Abstract

A weak (< 1000 G) magnetic field can influence photochemical processes through its effect on electron spin dynamics in a photogenerated radical pair. In a solution of pyrene and dimethylaniline this effect manifests as magnetic field-dependent exciplex fluorescence. Here we describe magnetofluorescence imaging (MFI). A localized magnetic null defines a fluorescence detection volume, which is scanned through a sample to create an image. MFI forms an image without lenses and in the presence of arbitrarily strong optical scattering. The resolution of MFI is in principle not limited by optical diffraction, although the present implementation is far from the diffraction limit.

Figures (4)

Apparatus for magnetofluorescence imaging. (a) The sample is immersed in a solution of pyrene/DMA (yellow disk) and placed in an octupole magnet. UV illumination impinges from above and fluorescence is sent via 10 mm acrylic light guide to a photomultiplier (PMT). Bias coils dither the location of the magnetic null at 870 Hz for lock-in detection. A mechanical x-y stage scans the magnet assembly relative to the sample. (b) Simulation of the magnetic field strength due to the permanent magnets in the plane z = 0. (c) Predicted point spread function based on field profile from (b), and the MFE on fluorescence from Fig. 1(b). (d) (Media 2, Media 3) Direct optical imaging of the point spread function. The light guide and PMT were replaced by a CCD camera. The sample chamber was filled with pyrene/DMA and the exciplex fluorescence was imaged onto the camera. The octupole magnets were scanned across a 7 x 7 grid. The dark spots correspond to the locations of the null in the magnetic field. The point spread function has a FWHM of 0.94 mm.

Magnetic field effect on fluorescence of a solution of pyrene/dimethylaniline (DMA). (a) Top: Bloch spheres for two electron spins in the radical pair. Absorption of a photon induces electron transfer from DMA to pyrene, to generate a spin-correlated radical pair. The electrons start in a singlet state and each precesses around its local effective magnetic field, which has contributions from hyperfine interactions and an applied magnetic field. Bottom: schematic showing the magnetically active nuclei, each drawn as a classical magnetic dipole. The hyperfine fields combine to yield a random, approximately static effective magnetic field. (b) (Media 1) Fluorescence intensity as a function of external magnetic field. The MFE depends only on the magnitude of B, not its direction.

Imaging through a scattering medium. (a) Sample cell. A glass object to be imaged (top) is placed in a chamber with a solution of pyrene/DMA (middle). The bottom and top of the chamber are blocked by ground glass plates so the object is obscured (bottom). (b) Magnetofluorescence images of the sample hidden inside the chamber. The vectors represent the local gradient of the MFE, which is only large at the solution / object interface. The quiver plot shows the boundaries of the object in (a). Inset: projected shadow image. (c) Quiver plot showing the boundaries of a single glass rod. Inset: projected shadow image.

MFE for prompt and delayed fluorescence of pyrene/DMA in degassed THF/DMSO (82%:18%). The uncertainties in the measurements are from photon shot noise. Error bars are shown for the delayed fluorescence measurements, and are negligible for the prompt fluorescence measurements.